A muscle strain is the stretching or tearing of muscle fibers. Most muscle strains happen for one of two reasons: either the muscle has been stretched beyond its limits or it has been forced to contract too strongly. In mild cases, only a few muscle fibers are stretched or torn, and the muscle remains intact and strong. In severe cases, however, the strained muscle may be torn and unable to function properly. To help simplify diagnosis and treatment, doctors often classify muscle strains into three grades, depending on the severity of muscle fiber damage:
Grade I strain. In this mild strain, only a few muscle fibers are stretched or torn. Although the injured muscle is tender and painful, it has normal strength.
Grade II strain. This is a moderate strain, with a greater number of injured fibers and more severe muscle pain and tenderness. There is also mild swelling, noticeable loss of strength and sometimes a bruise.
Grade III strain. This strain tears the muscle all the way through, sometimes causing a "pop" sensation as the muscle rips into two separate pieces or shears away from its tendon. Grade III strains are serious injuries that cause complete loss of muscle function, as well as considerable pain, swelling, tenderness and discoloration. Because Grade III strains usually cause a sharp break in the normal outline of the muscle, there may be an obvious "dent" or "gap" under the skin where the ripped pieces of muscle have come apart.
Although the risk of muscle strain is especially high during sports activities, you also can strain a muscle by lifting a heavy carton or by simply stepping off a curb.
Almost all types of athletic activity carry some risk of muscle strains, but these injuries tend to happen most often in contact sports, such as football, and in sports that require quick starts, such as basketball and tennis.